Question

Ovulation occurs in which of the following stages of an estrous cycle?
A. Proestrus
B. Estrus
C. Metestrus
D. All of the above

Answer 1

The estrous cycle is the reproductive cycle in females of non-primate vertebrates. Examples include rats, horses, pigs, dogs etc. Estrous cycle is induced by reproductive hormones after sexual maturity in females. Anestrous stages and pregnancies cause them to disrupt. Estrous cycle contains four phases namely proestrus, estrus, metestrus and diestrus.

Complete answer:
Option A Proestrus: It is the first stage of the estrous cycle. This stage is characterized by the development of endometrium and ovarian follicles. Endometrium develops with the help of estrogen. Some animals may experience bloody vaginal secretions. Ovulation does not occur in the Proestrus phase.
Option A is incorrect.
Option B Estrus: Estrus is a phase where the female becomes sexually receptive. It is also known as estrum or oestrum. During this phase, ovarian follicles mature under the influence of gonadotropic hormones followed by ovulation. Pheromones are also secreted in the estrus phase which attracts the male animals.
Option B is correct.
Option C Metestrus: It is the third stage of the estrous cycle characterized by sexual inactivity and the formation of the corpus luteum. The corpus luteum produces progesterone and the activity of estrogen subsides. In the absence of pregnancy the estrous cycle terminates with the regression of the corpus luteum.
Option C is incorrect.
Option D All of the above: This option is incorrect because ovulation occurs only during the estrus phase of the estrous cycle.
Option D is incorrect.

Hence, option B. Estrus is the correct answer.

Note:
Anestrus is a phase where the sexual cycle is at rest. It is a seasonal event regulated by melatonin released by the pineal gland. Melatonin represses stimulation of reproduction. Anestrus is induced during pregnancy, lactation, illness and energy deficit. The estrous cycle varies among species. The cycle is more frequent in smaller animals. For example, cats undergo an estrous cycle of $3 - 7$ weeks. Domestication of animals may affect the estrous cycle due to changes in the environment.